Since the start of the coronavirus outbreak and its subsequent spread around the world, infecting more than 13.8 million people so far, reports have also emerged of cases in animals.

From domestic cats to big cats, dogs and even mink, animals have tested positive for the virus in several countries. But a lot is still unknown about which species can catch it, and whether animals can catch the virus from and transmit the virus to humans.

The latest case of a mass COVID-19 infection of animals happened on a mink farm in Puebla de Valverde in northeastern Spain. Tests confirmed that 87 percent of the mink on the farm had contracted the virus, leading the agriculture minister for the Aragon region, Joaquin Olona, to order the culling of nearly 100,000 mink on the farm on Thursday "to avoid the risk of human transmission."

Seven workers have tested positive since the farm started being monitored on 22 May and officials suspect the virus first reached the farm through a worker who passed it to the animals. But Olona said it was not completely clear if "transmission was possible from animals to humans and vice versa."

This wasn't the first case of a coronavirus outbreak in a mink farm. In the Netherlands, tens of thousands of mink have been slaughtered since the start of the pandemic after 20 farms were found to be infected, the Dutch authorities said earlier this month.

In May, several workers in the Dutch mink farms were found to be infected with COVID-19. Authorities said research showed it was likely that on at least one of these farms, transmission occurred from mink to human.

Dutch ministers Hugo De Jonge and Carola Schouten wrote in a letter to parliament that there could be two cases of COVID-19 being passed from mink to human at infected farms, with the earlier case being referred to by the World Health Organization (WHO) as possibly the "first known cases of animal-to-human transmission."

But scientists say much more research needs to be conducted to identify whether animal-to-human transmission of COVID-19 is a widely present possibility and potentially high-risk.

The WHO said, however, that there have been some instances of COVID-19 patients infecting their pets, or human-to-animal transmission. "A number of susceptibility studies have shown that other animal species are also susceptible to the virus and can be infected, including cats, ferrets," it said.

One of the earliest reports of a pet being infected came from Hong Kong in late February this year. A dog kept in the same household as a confirmed COVID-19 patient had also tested positive for the virus after samples were analyzed.

Similar incidents were uncovered in more pet dogs, suggesting owners infected them, as researchers who studied the animals and members of the infected households found that an analysis of viral genetic sequences from the animals showed them to be identical to those in infected people. 

However, researchers said the study showed no evidence that dogs can pass the infection to other dogs or people, but it is impossible to be certain in which direction the virus traveled.

There have also been more common cases of infected felines, including domestic as well as big cats.

According to research published by the National Center for Biotechnology Information, COVID-19 replicated poorly in dogs, pigs, chickens, and ducks, but ferrets and cats could get infected. Cats were also found to be susceptible to airborne transmission.

One of the early cases of big cats becoming infected was reported in early April, when a tiger at New York's Bronx Zoo tested positive after exhibiting respiratory signs of the virus. A further four tigers and three lions were later confirmed as having COVID-19.

Later the same month, two pet cats in New York state tested positive for the coronavirus, marking the first confirmed cases in companion animals in the U.S., federal officials said. The cats were thought to have contracted the virus from people in their households or neighborhoods, according to the U.S. Department of Agriculture and the federal Centers for Disease Control and Prevention.

Several scientists also worry that the virus could jump back and forth between cats and people, because the animals often roam between households. But, although experimental evidence suggests cats can infect other felines, so far there have been no reports of cats infecting people.

So, although some studies show a high possibility of humans passing on the virus to certain limited species of animals, even if these animals don't show any symptoms, there is much less evidence of animals passing the virus to humans.

But scientists do agree the possibility should not be excluded and caution should be taken while further research is carried out.